For several winters, since the waistline dropped to the hips, our cities have been thronged with buxom young girls baring wobbling, bluish slabs of goosepimpled midriff. To be a teenager means that you must wear what fashion tells you to wear, whether it suits you or not. I exposed fat, dimpled knees below miniskirts; they wear their jeans around their upholstered pelvic bone, revealing several inches of gap until the top starts its diminutive reach.

I used not to know where they bought their clothes. Nowhere I shop sells anything like that above size 12 and at the cheaper end of the market, such as Topshop or River Island, the clothes end at size 16, and skimpy 16s at that: 16s for women who are 14 everywhere else.

There are shops that evade your attention because you don't need to know about them, then you discover you are out on a little self-absorbed twig of the big tree of British retailing. You don't have a clue. For example, I was standing at the bus stop a few weeks ago when I overheard a spat between two women, one of whom was telling the other that since she had lost weight she no longer had to buy "cheap tat" at New Look. "I like New Look," the other said, and there followed an accusing exchange about the limited means of single parents who work in chip shops versus the snobby ways of career girls.

According to its website, a quarter of British women have bought something at New Look, which isn't surprising, because they are everywhere. There are more than 500 branches in Britain and a couple of hundred more on the continent. The chain was founded in Taunton in 1969 by Tom Singh, whose grandparents came to Britain from India in 1950 and went door to door selling from a suitcase. Singh has proved conclusively that the pile-it-high, sell-it-cheap retailing strategy works, but New Look does more than that. It claims to be able to fix you up with something to wear whatever your age or size.

Last year New Look opened its flagship store on Oxford Street with a silver staircase designed by Future Systems which has lent the chain what the Times called "an air of sophisticated wit", and it has just opened a second flagship store at the Metro Centre in Gateshead, the largest mall in Europe. But New Look's real identity is to be found in the small shops in Britain's small towns, where they serve people on the lowest incomes who still want to kit themselves out in this season's trends. The locations of its London shops are a list of the impoverished boroughs of the city where gentrification has not yet quite squeezed out the low-paid.

On Saturday morning I went to the store on Brixton Road, where I lived for many years. Brixton has always struggled to attract high-street chains; Body Shop opened its doors only after a long campaign from the council. While Morley's department store houses franchises of Warehouse, Topshop, Dorothy Perkins and French Connection, only New Look has rooted itself into the high street with its own store. Stencilled on the widows are the words SHOP SHOP SHOP, and you can't make your message much clearer than that. There is a lot of stock, and you can buy a suit for the office for £50, a sequined party dress for £40, a silver evening bag for £5, a gauzy top for £16, a fake fur jacket for £45, a pink tweed coat for £55, a faux-fur collar with "diamond" brooch for £12, a jewelled silver cross with matching earrings for £5, a leather handbag for £8, and, in the Christmas present section, a pink jewelled dog collar and matching dog blanket for £12.

Like all the high-street stores, New Look has the ubiquitous wrap dress - though oddly with a dropped waist like a 20s flapper would wear - but other season's trends, such as tweed, seem to be missing. You can buy a pair of shoes or boots for £25 which give a very small nod in the direction of fashion, with round toes and T-bar straps, but they still look like something your mum would buy for you and which would make you cry with disappointment when you opened the wrapping paper.

For many years I had observed the women of Brixton, and the heroic attempts of its big-bosomed, big-bummed young women to shake their asses at the likes of Topshop and its narrow-hipped efforts to hem them in and close them off from all life's glamorous possibilities. They sparkled and spangled and made basic black look like what it was, a preparation for the grave. New Look appears to be the only high-street brand that really makes an effort to dress larger women, instead of ghettoising them into specialist stores. Many of the clothes on the racks reach size 18, which means that up to a certain point larger women are not segregated away from the main lines, although sizes above this have their own section, encouragingly at the front of the store and not hidden away at the back.

A few hours later, passing down Oxford Street on the bus, looking into Zara, I understood the key factor that insinuates inflated ideas about clothes that they don't actually possess: lighting. New Look illuminates its racks with fluorescent strips under whose harsh glare everything looks cheap and badly made. I'm not sure if Zara's clothes are better quality; but they look as though they are because they are better lit.

The Guardian had given me £50 to spend in New Look and it was pushing it to find anything I could honestly say I wanted. The hem of the pink tweed coat looked lumpy and uneven in the mirror. The faux-fur jackets were nice, but I never wear anything that ends at the waist. I bought a black pinstriped jacket for £35, and it was an achievement to spend that much. I haven't worn it yet and will be interested when I do to see whether it is mistaken for something more expensive.

You have to honour the company for extending its reach deep into the most unfashionable parts of Britain and giving their populations a crack at looking good within a severely limited budget. But my belief that its prices can't be beaten turned out not to be true. H&M, which makes better-quality, more fashionable clothes, undercuts it consistently. Its faux-fur collar is £7.99. Its jewelled evening bag is £5.99. Its red velvet jacket is £29.99. And though it has fewer outlets than New Look, they are based in every urban centre in England, Scotland and Wales, reachable from most small towns. What H&M doesn't do, and which almost no one on the high street will contemplate, is extend its size range beyond 16. And so you're left with New Look. What's your alternative? A sewing machine?

The lowdown

New Look

Most expensive item: Pink tweed coat, £55

Cheapest item: Two hair ornaments for £1

Sales staff: No one at the counter when I went to pay; waited several minutes. Very large security guard omnipresent